In laser vibrometry, by measuring vibration of an object at multiple points on a surface of the object, a vibration image of the object can be obtained. Applications for vibration imaging include acoustic detection of buried mines, non-destructive testing, structural health monitoring, modal analysis, etc.
In at least some known vibration imaging systems, a multiple beam laser Doppler vibrometer (MB LDV) is used to obtain a vibration image of an object. For a MB LDV, a laser beam is split into two identical arrays of beams: an array of object beams and an array of reference beams. The object beams are incident onto an object to form a grid of points on the object surface. Light scattered back from each point is mixed with a corresponding reference beam on an array of photodetectors, thus producing a plurality of interference signals. Each interference signal carries information about the object vibration in the corresponding points on the object surface. Simultaneous processing of the interferences signals facilitates generating a vibration image of the object.
Although MB LDV can provide remote vibration measurements with a high sensitivity and accuracy, MB LDV systems typically require operation on a stable platform. Specifically, platform motion can cause variation of the interference signals which is indistinguishable from signals caused by object vibration. In additional, motion of the platform or measured object along a direction of the laser beam (e.g., when the MB LDV operates onboard a moving vehicle) can disrupt the vibration measurements if the relative speed between the MB LDV and the object exceeds a maximum velocity measured by the MB LDV. The measurements in an MB LDV system may also be affected by air turbulence, which changes a phase of the object-reflected light. As a result, MB LDV applications are generally restricted to stationary platforms and short measurement distances. Accordingly, there is a desire for a vibration imaging sensor that has a relatively low sensitivity to platform motion and air turbulence.